You can see that there was 1.2 mm of rain (rain last hour 2100-2200) but the daily total changed from 1.4 mm to 2.8 mm. Looking at the rainfall graph, there was a "tip" between 2100 and 2101. It looks as if the tip was not attributed to the hourly total at 2200 UTC, and was in effect lost. Daily total was correct at both 2100 UTC and 2200 UTC.

I think this is similar to your other recent query. Cumulus maintains a list of readings for the last hour, adding a new one each minute and deleting those which are older than an hour ago. It determines the rainfall in the last hour from the difference between the rain counter in the first entry in the list and the last entry. So depending on the timing of when a rain tip occurs and when the the actual comparison is done, it is possible that the tip occurred slightly more than one hour ago, but at the previous comparison it hadn't occurred yet. Bear in mind that all of this happens in separate threads, so sometimes actions can occur in a different order.

I'll admit that I haven't totally convinced myself that the explanation is as above, but without seeing the data readings and the timing of exactly when Cumulus received the reading readings relative to when it updated the list and when it actioned the web tag in your twitter.txt file, it I can't come up with anything better.

That was my first thought too - a question of timing. But the tip didn't register in the previous hour's total either, so it was effectively lost from the hourly totals from the day. That was the bit that caught my attention as I couldn't account for the tip at all. I'm happy, though, as it wasn't lost from the daily total - but thought you'd like to be made aware of the anomoly.